Grid-controlled gas-filled tube



- April 27, 193?" J. SLEPMAN 2@78,883

GRID CONTROLLED GAS-FILLED TUBE Filed May 27, 1933 Load 6 INVENTOR Jose p77 Slepz'arz.

BY W

ATTRN Y Patented Apr. 27, 1937 UNITED STATES PATENT Fries GRID-CONTROLLED GAS-FILLED TUBE sylvania Application May 27, 1933, Serial No. 673,239

14 Claims.

My invention relates to grid-controlled electrical gaseous discharge tubes and particularly to such tubes when they are used for converting from direct current to alternating current.

One object of my invention is to provide an arrangement for insuring continuity and reliability in the functioning of control grids of gasfilled electrical discharge tubes.

Another object of my invention is to provide an arrangement for preventing back fires on the control grids of gaseous electrical discharge tubes.

Other objects of my invention will become apparent from reading the following description taken in connection with the drawing, in which Figure 1 illustrates a grid-controlled mercury arc tube in operative combination with electrical circuits embodying my invention; and

Fig. 2 illustrates a modification of Fig. 1 in regard to the grid connection.

Where an auxiliary electrode is interposed in the path of current flow through a gas between a pair of main electrodes, at least one of which is an emitter of electrons, current flow between 2 the main electrodes is, generally speaking, prevented from being initiated if the auxiliary electrode has a voltage lower than a certain critical value relative to the more negative of the two main electrodes. However, I have found that the control grid fails in effecting this preventive function, apparently due to discharges, or as they are frequently termed back fires, taking place between the auxiliary electrodes and one of the main electrodes. Such back fires appear to constitute voltage breakdown effects through the gaseous atmosphere, to be more or less random in their nature, and to be of relatively short duration. I have found that, if the current how 40 to the auxiliary electrode during this back firing discharge is kept to a relatively small value, of the order of a few milliamperes, the loss of discharge controlling eifect by the electrode is limited to the short duration of the back firing cause, and

45 normal conditions of operation, including the discharge-preventing function of the control electrode, are restored as soon as this brief period is over.

My invention, accordingly, comprises the em- 50 ployment of arrangements to limit current flow between the auxiliary electrode and at least one of the principal electrodes during these back firing periods; and as one embodiment I disclose in the following the employment of a thermionic 55 highv vacuum tube connected between the control electrode and the cathode, to limit back firing currents without substantially preventing the normal controlling function of the auxiliary electrade; and preferably provide such a thermionic tube with a high resistance shunt to improve its 5 protective operation.

Referring in detail to the figures of the drawing, which are duplicates except that the control grid is connected to the cathode in Fig. 1 and to the anode in Fig. 2, a gaseous electrical dis- 10 charge tube l, which may be of the well known mercury arc type, having a mercury cathode 2 with an exciting anode 3 in a conventional connection, is provided with main anodes i of con ventional type and auxiliary or control electrodes 5 also of conventional form. To illustrate one useful employment of my invention, I show the above-described discharge tube connected to convert energy from a direct-current circuit 6 to an alternating-current load circuit 1, or vice versa. One convenient arrangement of the alternatingcurrent circuit provides an inductance 8, between the neutral 9 of which and the cathode 2 of the discharge tube the direct-current circuit 5 is connected through an inductance II. The opposite terminals of the inductance 8 are connected to the main anodes 4 of the discharge device l through ancillary inductances H. A condenser l3 or other energy-storing device adapted to assist in determining the frequency of the alternating-current circuit 1 may be connected between the end terminals of the inductance 8.

Each auxiliary electrode 5 is connected to the cathode 2 through a source of grid-control voltage M, which may be of any desired wave form, and a high-vacuum discharge device !5 having a thermionically emissive electrode l6 and a cooperating electrode H. For most purposes, it is preferable to shunt a high resistance is across the terminals of the vacuum device iii. The conductivity of the rectifier it in its conductive direction should be sufiicient to maintain the control electrode 5 at the necessary degree of positive potential to promote current flow from an ode 4 notwithstanding the fact that an appreciably larger current may fiow from cathode 2 to electrode 5 at such times than at times when electrode 5 is in its negative potential phase.

While I do not wish to be limited in all respects by the correctness of the following theory, it is my belief that the discharge tube I5, having the property of readily passing in one direction currents of the magnitude corresponding to the electron emission of its cathode, will in the course of normal operation pass the necessary electric charges to permit the control electrode to render the space between the cathode 2 and the anode 4 5 readily conductive when the electrode 5 is positive relative to cathode 2 and its normal inverse leakage current will suflice to permit the control source I4 to charge the electrode 5 to a suflicient negative potential to prevent starting of the are between the electrodes 2 and 4 once it has been extinguished. On the other hand, back fires or discharges between the cathode 2 and the electrode 5 are most likely to occur when the latter is negatively charged; and the tube I5 opposes a high resistance to the flow of such currents, particularly if the electrode I1 is non-emissive. Any

breakdown of the gas within the discharge tube I between the cathode 2, or an anode 4, and the control electrode 5 is, accordingly, limited to a very small current value and its effects likewise limited to the very transient condition within the gas which occasioned the breakdown.

The above-described action of the discharge tube I5 may be regulated and even improved if a high resistance I8 is shunted across its terminals. This resistance should have such a value that the sum of the normal inverse leakage current of rectifier I5 and the current flowing through it, when the electrode 5 is at the maximum voltage it is expected to assume, is of the order of a few milliamperes; that is to say, of such a value as not to greatly disturb conditions between electrodes 5 and 2 over a time of more than a few microseconds. An arrangement in which the anode of tube I5 is also heated, but in such wise that its electron emission is limited to a reverse current of the same magnitude as would flow were a cold anode tube shunted by the resistor I8,

- might also be employed.

40 To give a very specific illustration of my invention, the tube I may be a mercury arc tube rated to withstand an inverse voltage between cathode and anode of 2000 volts and to have an output current capacity of 20 amperes. The voltage of the direct-current circuit 6 may be of approximately 800 volts and the inductance I I may be of 2 henrys; the inductance 8 may be of 0.1 henrys and the auxiliary inductances I2 of .01 henrys. The condenser I3 may be of 10 farads and the circuit I of 1000 terminal volts, the control voltage source I4 may be an alternating-current source of 100 volts; the discharge tube I5 may be a high vacuum two-electrode tube having a voltage rating of 500 volts and a current rating of 10 milliamperes and the resistor I8 having a value of 10,000 ohms.

One mode of operation of the above-described arrangement is as follows: Current flowing from the direct-current circuit 6 may flow through the right-hand half of the inductance 8 to the anode 4, the voltage source I4 impressing on the righthand control electrode 5 a positive potential while a negative potential is impressed on the left-hand control electrode 5. Because of the mutual reactions of the inductance 8 and the condenser I3, the right-hand electrode 4 will gradually drop in potential to a value negative to the cathode 2 and at the same time the left-hand anode 4 will become positive in potential. The control voltage sources I4 are so arranged that the left-hand control electrode 5 soon assumes a positive potential relative to cathode 2 and the right-hand control electrode 5 assumes a negative potential relative to the cathode 2. Current flow from the direct-current circuit 6 then shifts to the lefthand side of the inductance 8, thence to the lefthand anode 4 through the cathode 2 and back to the other side of circuit 6; and current flow ceases from the right-hand anode 4. The condenser I3 soon charges up to a reverse potential, thereby reducing the potential of left-hand anode 4 and increasing that of right-hand anode 4; and as soon thereafter as the potentials of control electrodes 5 relative to the cathode 2 reverse current flow through left-hand anode 4 ceases and is resumed through right-hand anode 4. Accordingly, an alternating potential is produced across the terminals of condenser I3 and of the load circuit 1.

Should a discharge take place between cathode 2 and either control electrode 5 while the latter is negatively charged and exercising its function of preventing current flow to its associated anode 4, the electron tube I5 and its associated resistor I8 will limit the current to so low a value, in accordance with the foregoing disclosure, as will prevent a permanent are from forming with its cathode on the control electrode 5. If such an arc were permitted to form at the time when the electrode 5 is first made negative, this arc would continue on to the time when the electrode should be exercising its blocking function, and the continuance of this are would prevent the electrode from blocking the starting of an arc to the anodes 4.

It will be. recognized that, while I have described my invention specifically in connection with an arrangement in which direct-current is derived from the circuit 6 and energy translated to the circuit I, the functioning of the tube I5 and resistance I8 is safeguarding the operation of the control electrode 5 will still apply where the control electrodes 5 are used to govern the translation of energy from the circuit I to the direct-current circuit 8; in other words, where the tube I is acting as a grid-controlled rectifier. It will also be operative if the circuit 6 is alternating current instead of direct current.

It will likewise be understood that my invention is not limited to control electrodes of mercury arc discharge tubes but tubes in which the electrode 2 is a thermionically emissive cathode, or other electron emitter, are within the purview of my invention. In fact, the arrangement of tube I5 is applicable to insurance 01' the proper functioning of control electrodes in gaseous discharge tubes generally.

It will also be understood that, while the thermionic vacuum tube I5 is a desirable arrangement for limiting current flow to the control electrode 5, other circuit elements which readily permit the flow of currents up to a certain critical value and oppose an increased impedance to currents of substantially larger magnitude are within the purview of my invention as substitutes for the arrangement I5, I8. Gas-discharge or other rectifiers are contemplated as substitutes for rectifier I5, as are also saturated core reactors such as those described in my case. It will be noted that if on the average rectifier I5 or rectifler 2-5 fails to rectify for 10 seconds out of every 10 minute period--rectifier I5 will fail to nullify the effect of back discharge to electrode 5 only once every times 10 minutes. An arrangement wherein control source I4 consists of transformers with a primary fed from the alternating current circuit 8, for example by shunting said primary in series with a capacitance across said alternating current line is within the purview of my invention. My application Serial No. 680,396 shows other desirable arrangements which make the control electrode voltage become positive to the cathode before the anode 4 changes from negative to positive potential; i. e. which cause the control electrode voltage to lead in phase the anode potential.

In accordance with the patent statutes, I have described a particular embodiment of my invention, but it will be recognized that this is for purposes of illustration and the broad principles thereof will be applicable in many other ways which will be evident to those skilled in the art. I, accordingly, desire the following claims to be given the broadest interpretation of which their terms are susceptible in view of the prior art.

I claim as my invention:

1. An electric discharge device having main and control electrodes, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to Vary the potential of said control electrode relative to one of said main electrodes, a high impedance shunted by a rectifier connected in series with said control electrode and one of said main electrodes.

2. An electric discharge device having main and control electrodes, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to vary the potential of said control electrode relative to one of said main electrodes, a high resistance shunted by a vacuum tube with an electron-emissive cathode connected in series with said control electrode and one of said main electrodes.

3. An electric discharge device having main and control electrodes, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to Vary the potential of said control electrode relative to one of said main electrodes, a high resistance shunted by a rectifier connected in series with said control electrode and one of said main electrodes.

4. An electric discharge device having main and control electrodes and an excited cathode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to vary the potential of said control electrode relative to one of said main electrodes, a circuit comprising a vacuum tube with an electron-emissive cathode connected in series with said control electrode and one of said main electrodes,

said circuit having a conductivity greater than zero in the normally non-conducting direction.

5. An electric discharge device having main and control electrodes and an excited cathode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to vary the potential of said control electrode relative to one of; said main electrodes, a high impedance shunted by a rectifier connected in series with said control electrode and one of said main electrodes.

6. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to vary the potential of said control electrode relative to one of said main electrodes, a high impedance shunted by a vacuum tube with an electron-emissive cathode connected in series with said control electrode and one of said main electrodes.

7. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to vary the potential of said control electrode relative to one of said main electrodes, a high impedance shunted by a rectifier connected in series with said control electrode and one of said main electrodes.

8. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to impose a periodic voltage between said control electrode and one of said main electrodes, a high impedance shunted by a vacuum tube with an electron-emissive cathode connected in series with said control electrode and one of said main electrodes.

9. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to impose a periodic voltage between said control electrode and one of said main electrodes, a high impedance shunted by a rectifier connected in series with said control electrode and one of said main electrodes.

10. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to impose a periodic electromotive force energized from said alternating-current circuit between said control electrode and one of said main electrodes, a circuit comprising a vacuum tube with an electronemissive cathode connected in series with said control electrode and the last-said main elec trode, said circuit having a conductivity greater than zero in the normally non-conducting direction.

11. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct-current circuit, an alternating current circuit connected toderive energy from the current passing between said main electrodes, means to impose a periodic electromotive force energized from said alternating-current circuit between said control electrode and one of said main electrodes, a high impedance shunted by a rectifier connected in series with said control electrode and the lastsaid main electrode.

12. A gaseous electrical discharge device having an excited cathode and an anode as main electrodes and a control electrode, a direct current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, an inductive coupling between the circuit of said anode and the circuit of said control electrode, a circuit comprising a vacuum tube with an electron-emissive cathode connected in series with said control electrode and one of said main electrodes, said circuit hav- 14. An electric discharge device having main electrodes and at least one control electrode, a direct-current circuit, an alternating current circuit connected to derive energy from the current passing between said main electrodes, means to vary the potential of said control electrode relative to one of said main electrodes, a rectifier connected between a control electrode and one of said main electrodes, at least one other path between said last-mentioned electrodes alternative to said rectifier, said other path including a high resistance.

JOSEPH SLEPIAN, 

